Piston or adjusatable DI for AR10?

I'm leaning towards adjustable gas block.

No need for either one unless you intend on suppressing it. A properly set up 308 AR (its not an AR10 unless you buy an Armalite) with a 5.4 oz buffer, proper spring and correctly sized gas port should run anything you want to put through it. An AGB will help with suppressed cycling but would probably be set on one position and left there if no suppressor is used. My AR10 comment may seem nitpicky but Armalite has the only rights to use the term and there are compatability issues if you try to mix and match Armalite pattern and DPMS pattern parts. Even with DPMS pattern rifles there are parts differences from one manufacturer to another. There is no Milspec on these large frame ARs and any company can make parts any way they wish and most end up fouling something up somewhere, whether its a too small gas port in the barrel, too light of a buffer weight, or too light buffer spring.

If you are going to build an AR.308 (only Armalites at AR10's) then most definitely get an adjustable GB as there is a very good chance that you will need it. If you are buying a rifle off the shelf you could likely do without.

Adjustable gas block, you can tame the recoil a bit with it. Plus a lot of piston stuff is proprietary so having a DPMS LR308 or AR10 style BCG makes life easier if parts break.